1. Field of the Invention
The present invention relates to an improved system for removing oxides of nitrogen from waste gas produced by various combustion processes.
2. Background of the Prior Art
When air consisting of nitrogen (N.sub.2) and oxygen (O.sub.2) is fed into a combustion process such as a boiler, an engine, or a turbine, an exhaust consisting primarily of N.sub.2, carbon dioxide (CO.sub.2) and water H.sub.2 O) is produced. Lesser products of the process are carbon monoxide (CO), unburned O.sub.2, unburned fuel, and oxides of nitrogen NO.sub.x. These oxides of nitrogen are undesirable because they are both corrosive and a source of air pollution. As a result, a substantial amount of research has centered about techniques for reducing the amount of NO.sub.x present in these waste gases. See: Bartok, et al, Control of NO.sub.x Emissions from Stationary Sources, Chemical Engineering Progress 64 (February 1971); Aldhart, et al, Processing Nitric Acid Tail Gas, Chemical Engineering Progress 73 (February 1971); Newman, Nitric Acid Plant Pollutants, ChemicaL Engineering Progress 79 (February 1971). In general, three techniques have involved either modifying the combustion process itself so as to reduce the amount of NO.sub.x produced or treating the waste gases whereby the NO.sub.x produced is removed or converted.
For accomplishing the latter of these techniques, NO.sub.x removal, a commonly practiced process is catalytic reduction. This process, described in all the above mentioned articles, is taught by, e.g., U.S. Pat. Nos. 3,846,981 to Paczkowski, issued Nov. 12, 1974; 3,826,810 to Lawson, issued July 30, 1974; 3,449,063 to Griffing, et al., issued June 10, 1969; 3,279,884 to Nonnenmacher, et al., issued Oct. 18, 1966; and 2,975,025 to Cohn, et al., issued Mar. 14, 1961.
Catalytic reduction has been accomplished in the prior art by adding a fuel gas, or reducing agent, to the waste gas in the presence of a catalyst (See, e.g., Nonnenmacher, et al., supra; Cohn, et al., supra; U.S. Pat. No. 3,232,885, issued Feb. 1, 1966 to Henke; Canadian Pat. No. 668,384, issued Aug. 13, 1963 to Henke; and Canadian Pat. No. 787,836, issued June 18, 1968 to Henke) or by merely adding the reducing agent to the gas at any point prior to passing the gas over the catalyst (See, Griffing, et al., supra). Catalytic reduction may be accomplished non-selectively, using a reducing agent such a methane, or selectively, using a reducing agent such as ammonia. In non-selective reduction, the agent reacts with gases other than NO.sub.x, especially oxygen as well as with NO.sub.x. In selective reduction, the agent reacts almost exclusively with NO.sub.x. Therefore, a greater quantity of reducing agent is necessary in non-selective reduction than is necessary in selective reduction. Furthermore, the reaction that occurs between the agent and oxygen in non-selective reduction produces a tremendous amount of heat that can destroy the catalyst used and can result in increased equipment and control costs.
In spite of these disadvantages, non-selective catalytic reduction has been practiced, especially in treatment of nitric acid plant exhaust gases, more commonly than has selective reduction (See, Newman, supra, at 81) because the cost of ammonia in the past has far exceeded the cost of methane.
Over the past few years, however, the cost differential between methane and ammonia has decreased substantially thus increasing the efforts to improve the process of selective catalytic reduction. These efforts have centered about finding suitable catalysts (See, Cohn, et al., supra, teaches use of platinum group metals; Nonnenmacher, et al., supra, teaches use of vanadium, molybdenum and tungsten; Griffing, et al., supra, teaches use of copper in oxide form) and determining optimum process conditions (See, Cohn, et al., supra, teaches a preferred range of approximately 320.degree. F. to 575.degree. F.; Nonnenmacher, et al., supra, teaches a preferred range of approximately 390.degree. F. to 662.degree. F.; Griffing, et al., supra, teaches a preferred range of 600.degree. F. to 800.degree. F.). In spite of these efforts, selective reduction of NO.sub.x, especially in nitric acid plants, has not been very successful (Bartok, et al., supra, at 70). As a result, in many locales, it is still more economical to use low efficiency non-selective reduction for removal of NO.sub.x from exhaust gases using methane as a fuel.